This invention relates to a positional deviation detecting method, and to an exposure apparatus or a device manufacturing method using the same. The present invention is particularly suitable for use in an exposure apparatus or a scan type exposure apparatus to be used in a lithographic process for the manufacture of microdevices such as semiconductor devices (e.g., IC or LSI), image pickup devices (e.g., CCD), display devices (e.g., liquid crystal panel) or magnetic heads, for example, for relative positioning or alignment between a first object such as a mask or reticle (hereinafter xe2x80x9cmaskxe2x80x9d) and a second object such as a wafer when a fine pattern such as an electronic circuit pattern formed on the first object is to be lithographically transferred to the second object.
In exposure apparatuses for device manufacture, relative alignment of a mask and a wafer is an important factor in improvement of performance. For a DRAM which is a representative semiconductor integrated circuit, an overall registration precision of about ⅓ to xc2xc of the minimum linewidth of a resolution pattern image is required. Particularly, in recent exposure apparatuses, an alignment precision of 20 nm or less is required to meet further enlargement of integration of semiconductor chips.
For a 1-Gbit DRAM currently being developed, an overall registration precision of 40 nm to 50 nm is required and, within it, the precision to be shared for the alignment precision will be 10-15 nm.
In many exposure apparatuses, a mask and a wafer are formed with positioning marks, called alignment marks. A positional deviation between these alignment marks is optically detected and, on the basis of a detected value, the positioning (alignment) of the mask and the wafer is performed. As for detection of alignment marks, there are a method in which a mark is optically enlarged and projected on a CCD by which image processing is performed; a method in which a straight diffraction grating is used as a mark and the phase of diffractive light produced thereby is measured; and a method in which a zone plate (grating lens) is used as a mark and light diffracted by the zone plate is detected upon a predetermined plane, whereby positional deviation of the diffracted light is detected.
Among these detection methods, the methods that use a straight diffraction grating or a zone plate as an alignment mark have a feature that, in the sense that detection is less influenced by any defect or fault of the mark, it is tough on the semiconductor process and enables relatively high precision alignment.
FIG. 1A is a schematic view of a position detecting system of a conventional type. In the drawing, parallel light emitted from a light source 72 passes through a half mirror 74 and, then, it is collected by a condenser lens 76 toward a convergence point 78. After this, the light illuminates a mask alignment pattern 68a upon the surface of a mask 68 and a wafer alignment pattern 60a upon the surface of a wafer 60. The alignment patterns 68a and 60a each comprises a reflection type zone plate which serves to define a spot of light convergence upon a plane which is orthogonal to the optical axis, passing through the convergence point 78. A deviation of light convergence spot position upon that plane is detected by a detector 82, with the light being guided thereto by the condenser lens 76 and another lens 80.
Control circuit 84 actuates a driving circuit 64 on the basis of an output signal from the detector 82, by which relative positioning of the mask 68 and the wafer 60 is performed. FIG. 1B is a schematic view for explaining the imaging relation of lights from the mask alignment pattern 68a and the wafer alignment pattern 60a, shown in FIG. 1A.
In FIG. 1B, a portion of the light divergently emitted from the light convergence point 78 is diffracted by the mask alignment pattern 68a, by which a light convergence point 78a, representing the position of the mask, is defined adjacent to the point 78. Also, another portion of the light passes through the mask 68 as zeroth order transmissive light and, with its wavefront unchanged, it impinges on the wafer alignment pattern 60a on the wafer 60 surface. After being diffracted by the pattern 60a, the light passes through the mask 68 again as zeroth order transmissive light, and it is collected in the neighborhood of the light convergence point 78, whereby a light convergence point 78b representing the position of the wafer is produced.
In FIG. 1B, when the light diffracted by the wafer 60 defines a light convergence spot, the mask 68 serves simply as a transparent member.
The position of the light convergence point 78b thus produced by the wafer alignment pattern 60a bears a deviation xcex94"sgr"xe2x80x2 corresponding to a deviation xcex94"sgr" of the wafer 60 with respect to the mask 68, in a direction (lateral direction) along the mask or wafer surface and along a plane perpendicular to the optical axis, passing through the light convergence point 78.
The amount of this deviation xcex94"sgr"xe2x80x2 is measured with reference to an absolute coordinate system defined upon a sensor, whereby the deviation xcex94"sgr" is detected.
Usually, for alignment of a mask and a wafer based on detection of a positional deviation therebetween, the mask and the wafer are controlled to be placed with a mutual spacing in a predetermined range and, thereafter, they are brought into alignment on the basis of positional information obtainable from a sensor by use of alignment patterns provided on the mask and the wafer.
Such a method, however, involves a problem that Fraunhofer diffraction light from openings of mask and wafer alignment marks enter a central portion of a sensor, to cause interference with signal light that results in a decreased signal-to-noise ratio of a produced alignment signal as well as non-linearity of a signal to the mask-to-wafer relative deviation.
The influence of such Fraunhofer diffraction light may be reduced by arranging, as shown in FIG. 1C, a wafer alignment mark WA with eccentricity with respect to a mask alignment mark MA, in a state where there is no positional deviation between a mask circuit pattern and a wafer circuit pattern. With this arrangement, since the Fraunhofer diffraction light from the openings is spatially separated from the signal light, the influence of interference is reduced and a good signal is produced.
With this method, however, since the wafer alignment mark WA has a shape asymmetrical with respect to the alignment detecting direction, as shown in FIG. 1C, there is a problem that asymmetrical non-uniformness of diffraction efficiency is easily produced within the alignment mark.
If the diffraction efficiency distribution within the mark is asymmetric, a beam spot of signal light produced on a sensor becomes asymmetric. It causes a shift of the peak position and gravity center position of the spot, which leads to a detection error. The degree of influence to asymmetry of a signal light spot of the asymmetry of diffraction efficiency distribution within the mark, becomes notable with a larger deviation of the mask-to-wafer spacing (gap) with respect to a design gap, since it causes defocus of the spot.
The non-uniformness of diffraction efficiency within the mark may be attributable to the fact that: since an alignment mark has a power with respect to the alignment direction, the pitch thereof (i.e., the linewidth thereof) changes within the pattern, whereas it is difficult to control the linewidth and pattern step height (level difference) over the whole wafer surface, with respect to every linewidth and throughout the etching process, the deposition process and so on of the device process.
Particularly, due to miniaturization of a circuit pattern, the linewidth of-a circuit pattern is 0.15 micron or less and the linewidth range of an alignment mark is widened, from a few microns to about 10 microns. Further, the diameter of a wafer is enlarged. In consideration of these tendencies, the problem of alignment detection error resulting from non-uniformness of linewidth or level difference of an alignment mark of a wafer will become serious.
Further, in addition to the signal light diffracted by a mask alignment mark and then by a wafer alignment mark and transmitted through a mask, there is unwanted diffractive light produced, which is transmitted through the mask alignment mark, diffracted by the wafer alignment mark, and then diffracted by the mask alignment mark. If the level difference of the wafer alignment mark varies, the intensity ratio between the signal light and such unwanted diffractive light changes, causing a detection error. Moreover, in relation to a global alignment process which is very advantageous with respect to throughput, there is a problem that any local distortion due to a wafer process cannot be corrected.
While the problems described above all concern deterioration of precision which is attributable to a semiconductor process, there is another problem of deterioration of registration precision which is caused by a large sensitivity of an alignment detection system. Even when an alignment mark is provided in good order, if the signal light has an angle xcex8 to the optical axis (FIG. 1D), due to a set gap error xcex4g of mask and wafer, the spot position displaces by tan(xcex8)xc2x7xcex4g, causing a detection error.
Deterioration of registration precision attributable to the sensitivity of the alignment detecting system such as gap variation or deterioration of registration precision resulting from deformation or local distortion of an alignment mark due to a process, as described above, is not a problem solely peculiar to an alignment method using a zone plate or grating lens but a problem also involved in the other alignment methods described hereinbefore.
It is an object of the present invention to provide a positional deviation detecting method in which appropriately set plural grating lenses each having a power in a positional deviation detecting direction (X direction) are provided on a first object (mask) and a second object (wafer), and in which relative positional deviation between the mask and the wafer can be detected very precisely on the basis of a deviation of incidence position upon a predetermined plane of light coming from the grating lenses of the mask and the wafer, such that high precision alignment is assured.
It is another object of the present invention to provide an exposure apparatus and/or a device manufacturing method which uses such a positional deviation detecting method.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.